It is third time she has won the race ,which surprised us al

这句话的完整句应该是这样的
It is third time （that） she has won the race
这是个定语从句,that指代she has won the race这件事情,that可以省略,
所以给你的感觉是有两个主语.
再问： 不是说that与which相似是代词，后面的从句要缺点什么吗？ she has won the race 没缺东西呀?
再答： 我明白你的意思，你说的少个词应该是这种情况 It is third time she has won the race ,which surprised us all 看这个句子，后面的这句，which surprised us all，如果去掉which，就少主语，是吧，所以which代的是主语，代的前面这件事。代词做主语时是不能省略的。 但是如果引导从句或者代宾语是可以省略的。 带宾语的我再给你举个例子 the apple which he bought 中which 带的是宾语(he bought）the apple就可以省略成 the apple he bought